Drought-induced changes in root hydraulic conductance (L P) and mercury-sensitive water transport were examined for distal (immature) and mid-root (mature) regions ofOpuntia acanthocarpa. During 45 d of soil drying,L P decreased by about 67% for distal and mid-root regions. After 8 d in rewetted soil,L P recovered to 60% of its initial value for both regions. Axial xylem hydraulic conductivity was only a minor limiter of L P. Under wet conditions, HgCl2 (50 μM), which is known to block membrane water-transport channels (aquaporins), decreasedL P and the radial hydraulic conductance for the stele (L R, S) of the distal root region by 32% and 41%, respectively; both L P andL R, S recovered fully after transfer to 2-mercaptoethanol (10 mM). In contrast, HgCl2 did not inhibit L P of the mid-root region under wet conditions, although it reducedL R, S by 41%. Under dry conditions, neitherL P nor L R, S of the two root regions was inhibited by HgCl2. After 8 d of rewetting, HgCl2 decreased L Pand L R, S of the distal region by 23% and 32%, respectively, but L P andL R, S of the mid-root region were unaltered. Changes in putative aquaporin activity accounted for about 38% of the reduction in L P in drying soil and for 61% of its recovery for the distal region 8 d after rewetting. In the stele, changes in aquaporin activity accounted for about 74% of the variable L R, S during drought and after rewetting. Thus, aquaporins are important for regulating water movement for roots of O. acanthocarpa.